This invention relates to line protectors for telephone and like communications circuits and to methods of making such protectors. The line protectors with which the present invention is concerned are of the type known generally as central office protectors and are utilized to prevent inside plant or central office equipment from damage due to overvoltage or overcurrent faults on incoming lines.
In order to comply with modern standards of line protection, it is becoming common to design line protectors with back-up surge voltage protection. Where gas tube surge voltage arresters are utilized in the protector, it is realized that occasionally a gas tube will be defective by being vented to atmosphere. Under such conditions the arc gap within the tube is usually too wide to be effective for surge voltage protection. Often the electrode gap in the gas tube is of the order of 0.030 to 0.040" and may have a voltage breakdown of several hundred volts when the gas tube is functioning properly. However, when the tube is vented to atmosphere the breakdown voltage may be of the order of several thousand volts. This voltage is much too high for satisfactory protection. As a result, back up air gaps external to the gas tube are being designed into the protector modules, and these air gaps are generally designed to break down in the order of 1000 volts or less. Such air gaps require a gap spacing of only a few thousandths of an inch, for example in the order of 0.0065" to 0.0075" for air gap between metal electrodes.
In the mass production of protector modules it is important that the secondary air gaps be accurately established, i.e. that the spacing of the electrode elements forming the secondary air gap are not too widely or too narrowly separated. Accordingly, with normal manufacturing tolerances it sometimes is a problem assuring that the secondary air gap is always within specified tolerances.